This invention relates to an electromagnetic coupling such as an electromagnetic clutch of the type having a stationary magnet core, a rotor and a relatively rotatable armature. The magnet core and the rotor are both defined by inner and outer radially spaced pole rings with the rings of the core being spaced from the rings of the rotor by radial air gaps of precise narrow width. A multiple turn winding is disposed between the core and the rotor and, when energized, produces magnetic flux which threads across the air gaps and draws the armature toward pole faces defined by the ends of the rings of the rotor. The armature thus is coupled to the rotor and rotates in unison with the rotor. The coupling torque is dependent in part upon the amount of controlled flux which is effectively transferred across the air gaps between the rotor and the magnet core.
A clutch of this general type is disclosed in Pierce U.S. Pat. No. 2,796,962. In that clutch, the outer ring of the rotor is formed with a circumferentially extending groove which receives an endless belt for driving the rotor.